1. Field of the Invention
The present invention relates to an optical recording medium and an optical recording/reproducing method using the same.
2. Related Art
As recording media for recording digital data, optical recording media such as CD (Compact Disc) and DVD (Digital Versatile Disc) have been widely used. These optical recording media can be broadly classified into the ROM-type optical recording media such as CD-ROM (Read Only Memory) and DVD-ROM where data is not added or rewritable, the write-once type optical recording media such as CD-R (Recordable) and DVD-R where data can be added but not rewritable, and the rewritable optical recording media such as CD-RW (Rewritable) and DVD-RW where data is rewritable.
As well known, in the ROM-type optical recording media, data is usually recorded in the form of pre-pits formed on the substrate during manufacturing. In the rewritable optical recording media, phase-change material, for example, is used as a material for the recording layer. In general, data is recorded by the use of a change in the optical characteristics caused by the phase change.
Meanwhile, in the write-once type optical recording media, organic dyes such as cyanine dyes, phthalocyanine dyes, and azo dyes are used in the recording layer. In general, data is recorded by the use of a change in the optical characteristics caused by its chemical change (occasionally, a physical change may occur along with the chemical change).
Since organic dyes degrade when exposed to sunlight, for example, it is not easy to improve long-term storage reliability of the medium using such an organic dye in the recording layer thereof. To improve long-term storage reliability of the write-once type optical recording media, it is preferable to make the recording layer with a material other than organic dyes. As an example that has formed the recording layer with a material other than organic dyes, there is a technique to laminate two reaction layers to form a recording layer, as disclosed in Japanese Patent Laid-Open Publication No. Sho 62-204442.
In recent years, the data recording density has been raised and some next-generation type optical recording media capable of transmitting data at a very high rate have been proposed. In such next-generation optical recording media, the spot size of the laser beam used for recording/reproducing data must be focused small to accomplish a high-capacity, high-speed data transmission rate. In order to make the beam spot smaller, the numerical aperture (NA) of the object lens that focuses the laser beam must be 0.7 or larger, for example, near 0.85, and at the same time the wavelength, λ, of the laser beam must be 450 nm or shorter, for example, near 400 nm.
On the other hand, if the NA of the object lens is raised to focus the laser beam, such a problem arises that the tolerance of warpage and tilt of the optical recording medium, namely, the tilt margin becomes very small. The tilt margin, T, can be expressed by the following Equation (1):T=λ/(d•NA3)  (1)where the wavelength of the laser beam used in data recording/reproducing is λ and the thickness of the light transmission layer (transparent substrate) working as the light path for the laser beam is d.
As the Equation (1) indicates, the tilt margin becomes smaller as the NA of the object lens grows. Meanwhile, the coefficient W of wave front aberration is expressed by the following Equation (2):W={d•(n2−1)•n2•sin θ•cos θ•(NA)2}/{2λ(n2−sin 2θ) 3/2}  (2)where the refractivity of the light transmission layer (transparent substrate) where the wave front aberration (coma aberration) arises is n and the tilt angle is θ.
As indicated by Equations (1) and (2), the thickness, d, of the light transmission layer (transparent substrate) where the laser beam for data recording/reproducing comes in must be small to effectively prevent coma aberration while ensuring a large tilt margin,
For these reasons, it is important in the next-generation optical recording media to thin the light transmission layer (transparent substrate) to about 100 μm for preventing coma aberration while ensuring a sufficient tilt margin. Thus, in the next-generation type optical recording media, different from the currently-used optical recording media such as CD and DVD, it is difficult to form a recording layer and the like on the light transmission layer (transparent substrate). Instead, such a technique is under investigation that forms a thin resin film as the light transmission layer (transparent substrate) by the spin coating and other methods on the recording layer and the like formed on the substrate. For this purpose, in the manufacturing of the next-generation optical recording media, films are sequentially deposited from the opposite side of the laser incident face, unlike the currently used optical recording media where the films are sequentially deposited from the light incident side.
However, a problem is found that when the recording layer is made of two reaction layers deposited on the substrate in the next-generation optical recording media the noise level is likely to become high (the C/N ratio becomes small) during signal restoration, compared with the conventional optical recording media such as CD and DVD where the recording layer formed in the light transmission layer (transparent substrate) is made of two reaction layers.
Meanwhile, to meet the recent growing needs for environmental protection, the recording layer of the optical recording medium should be made of materials of a smaller environmental burden. Furthermore, to improve the long-term storage reliability, the material for the recording layer of an optical recording medium should be sufficiently resistant to corrosion and degradation.